The study of events in early mouse development such as gastrulation and formation of the body axis have been greatly assisted by isolation of both mutations affecting such processes and genes whose products could regulate these developmental events. The search for more developmentally important genes would be aided by approaches that combine both mutational and molecular analysis. One such approach involves integration of "gene- trap" reporter vectors into mouse embryonic stem cells. These vectors only allow expression of the reporter gene when integrated within the transcription unit of a host gene. Integrations that faithfully reflect host gene expression, disrupt the host gene and allow its easy molecular cloning, can be preselected by reporter gene expression in vitro or in chimeras. The aims of this proposal are 1) to complete the molecular and mutant analysis of c. 40 already isolated cell lines with integrations that have spatially restricted expression around gastrulation; 2) carry out a detailed molecular and embryological analysis of one such integration whose expression is confined to the node and head process derivatives; 3) carry out an in vitro screen for gene trap insertions that show altered expression after treatment with the growth factor bFGF. These studies should provide considerable information on novel genes involved in patterning the mouse embryo and give insights into the ontogeny of human congenital birth defects and genetic diseases.